1. Field of the Invention:
The present invention relates to ferrite-austenite stainless steel castings having an improved erosion-corrosion resistance.
2. Description of the Prior Art:
Hitherto, stainless steel castings have widely been used as structural parts in machine and equipment for pollution prevention which are used in coke-sintering apparatus, copper refining apparatus, electric power apparatus, chemical plants, food processing equipment, marine related equipments and the like, for example, impeller, casing, front cover and valve of pumps for a wettype exhaust fume desulfurization system by lime-gypsum process. In such applications, however, it is known that the stainless steel castings are susceptible to erosion-corrosion in the presence of high-temperature and high-concentration sulfuric acid, sulfurous acid gas, chlorine and other halogen ions, sludge, clay, lime, gypsum and the like and also are susceptible to pitting corrosion due to the use of industrial water and sea water.
Recently, various stainless steel castings have been developed exhibiting a high corrosion resistance under the above mentioned severe conditions. For instance, the stainless steel castings JIS G 5121-SCS 11, SCS 14 (ASTM CF8M), Carpenter stainless No. 20 and Hastelloy C, which have a chemical composition shown in the following Table 1, are used as the material for structural parts of a pump.
Table 1 __________________________________________________________________________ Symbol of Chemical composition (%) material C Si Mn Cu Ni Cr Mo Co W __________________________________________________________________________ JIS- 5.00 23.00 1.50 SCS 11 .ltoreq.0.10 .ltoreq.1.50 .ltoreq.1.00 .about.7.00 .about.27.00 .about.2.50 JIS- 10.00 17.00 2.00 SCS 14 .ltoreq.0.08 .ltoreq.1.50 .ltoreq.2.00 .about.14.00 .about.20.00 .about.3.00 Carpenter 24.0 19.0 2.0 stainless .ltoreq.0.07 .ltoreq.1.0 .ltoreq.2.0 .about.30.0 .about.21.0 .about.3.0 No. 20 Hastelloy 15.5 16.0 3.75 C .ltoreq.0.12 .ltoreq.1.0 .ltoreq.1.0 bal. .about.19.5 .about.18.0 .ltoreq.2.5 .about.5.25 __________________________________________________________________________
SCS 11 and SCS 14 are comparatively cheap, but are low in strength and poor in corrosion resistance, particularly erosion-corrosion resistance. On the other hand, Carpenter stainless No. 20 is good with respect to resistance to sulfuric acid, but is expensive, low in strength and poor with respect to pitting-corrosion resistance and erosion-corrosion resistance. Further, Hastelloy C displays a good corrosion resistance, but is expensive and low in strength. Therefore, these stainless steel castings are not yet satisfactory for the above mentioned applications and may cause serious problems during use.